Conventional filter systems for waste water purification comprise a filtration unit with a tubular or box-shaped housing which is open at the top and bottom and in which a plurality of flat filter elements are arranged vertically and parallel to but spaced apart from one another. The spaces between the individual filter elements form passages through which medium can flow. The individual filter elements are configured as cushions or cartridges, in which a flexible drainage layer or rigid filter plate is surrounded on both sides by filtration membranes.
EP 0 707 884 A1 discloses an apparatus for filtering and separating in particular biologically organic flow media by reverse osmosis and micro-, ultra- and nanofiltration, having a pressure-tight housing, having an inlet for the flow medium and outlets for the retentate and the permeate, and a plurality of filter elements, which are accommodated in the housing, are spaced apart from one another, are designed in the form of a membrane cushion and have the flow medium flowing around them, a plurality of separate stacks of membrane cushions being arranged behind or next to one another in the housing, and the flow medium flowing around the stacks in series or in parallel.
EP 0 129 663 A1 discloses a membrane cushion for desalination of water by reverse osmosis, ultrafiltration, hyperfiltration, gas permeation and the like, in which a drainage layer is arranged between two outer membranes and the drainage layer is welded to the membranes continuously and in a pressure-tight manner in an edge zone.
WO 03/037489 A1 describes a filtration module for purifying waste water, having a plurality of filter membrane pockets, which include at least one opening for removing water from their interior and are arranged vertically, parallel to and preferably at an equal distance from one another in a rigid holder, in such a way that a liquid can flow intensively through the spaces between adjacent filter membrane pockets.
The known filter systems include either rigid housings and/or rigid membrane cartridges, which are complex and expensive to produce and assemble, increase the space taken up by the filter module, impede the flow of the liquid that is to be filtered and at which relatively coarse contaminants, such as for example hairs and fibers, tend to accumulate, thereby causing blockages.
When a filter system is operating, particles with a diameter which is too large to pass through the pores in the membrane are retained on the membrane surface and in some cases stick to it. The accumulation of these particles over prolonged periods of time causes the build-up of a filter cake which increasingly blocks the membrane surfaces and reduces the filter capacity of the system. The membrane surfaces are mechanically cleaned at regular intervals, including removal of the filter cake, for example by means of brushes and water jet, as part of plant maintenance. The housing of the known filter systems considerably restricts access to the membrane surfaces and thereby makes cleaning more difficult.
In addition to the mechanical cleaning, there is also the option of flushing the membrane pores clear by back-flushing, i.e. reversal of pressure. In the known areal filter systems, back-flushing is not generally used, since on the one hand it entails the risk of overstretching the filter element and causing cracks in the membrane, which is sensitive to tensile forces, and on the other hand the membranes of adjacent filter elements are pressed onto one another, thereby blocking the back-flow and the removal of the filter cake.
Some of the known filter systems have the additional drawback that the growth of filter cakes is locally accelerated on account of a spatially uneven distribution of the transmembrane differential pressure. The growth rate of the filter cake is directly proportional to the transmembrane volumetric flow and therefore to the transmembrane differential pressure. With regard to the liquid pressure, the known filter systems have three regions, referred to as the filter inlet, the filter element interior and the permeate outlet. In operation, a small pressure difference (Pv−Pa>0) is applied between the filter inlet (Pv) and permeate outlet (Pa), by means of suction pumps on the outlet side or pressure pumps on the inlet side, so that some of the liquid which is to be filtered flows from the filter inlet through the membrane to the permeate outlet. Under normal operating conditions, the flow velocity and pressure drop in the filter inlet and permeate outlet are low, so that substantially the constant pressures Pv and Pa act on the filter elements and the outflow bores. This does not apply to the filter element interior (Pi), in which the permeate flows quickly and moreover the flow velocity increases toward each outflow bore. Accordingly, a position-dependent static pressure Pi, where Pi is between Pa and Pv (Pa≦Pi≦Pv) and decreases toward each outflow bore, acts in the filter element interior. The volume of liquid which flows through the membrane per unit time and area is proportional to the transmembrane differential pressure Pv−Pi. Consequently, a filter cake builds up more quickly in regions with a high transmembrane differential pressure, i.e. in the vicinity of an outflow bore, than in regions further away. By way of example, the edge suction disclosed in WO 03/037489 A1 promotes the growth of filter cake at the edge of the filter element, with an associated premature drop in the filter capacity.